The rear-view mirror is a cipher with a built-in retrospect.

When encoding a message, each letter is encoded as the sum of the letter and the preceding letter in the message. Each letter has a value that corresponds to the position of the letter in the alphabet: A=0, B=1, C=2, …, Z=25. The addition is performed modulo 26 (the number of letters in the alphabet). For example, suppose we have to encode the letter D and that letter is preceded in the message by the letter Z, then we get \[ \text{D} + \text{Z} = (3 + 25)\ \text{mod}\ 26 = 2 = \text{C} \] The following rules also apply when encoding messages:

uppercase letters remain uppercase letters
lowercase letters remain lowercase letters
the first letter of the message has no preceding letter and remains in place
all characters that are no letters also remain in place

Decoding an encoded message is done by rolling back the encoding process. To decode a letter from the encoded message, the preceding letter in the original message is subtracted. Decoding must therefore be done from left to right. For example, suppose we have to decode the letter C and the preceding letter in the message has already been decoded as Z, then we get \[ \text{C} - \text{Z} = (2 - 25)\ \text{mod}\ 26 = 3 = \text{D} \]

Assignment

Write a function encode_letter that takes two strings (str): i) a letter $$L$$ from a message that must be encoded and ii) the preceding letter $$P$$ in the message. The function must return the encoded version (str) of the letter $$L$$.
Write a function decode_letter that takes two strings (str): i) a letter $$L$$ from an encoded message and ii) the preceding letter $$P$$ in the original message. The function must return the decoded version (str) of the letter $$L$$.
Write a function encode that takes a message (str). The function must return the encoded message (str).
Write a function decode that takes an encoded message (str). The function must return the original message (str).

Example

        >>> encode_letter('d', 'Z')
'c'
>>> encode_letter('T', 'e')
'X'

>>> decode_letter('c', 'Z')
'd'
>>> decode_letter('X', 'e')
'T'

>>> encode('Saw things clearer once you were in my rear-view mirror.')
'Ssw papvty unpervv fbpg cmi qavv mv zk pver-mdma iuziff.'
>>> encode('The past is always tense. The future perfect.')
'Tal tpsl ba slhwyq lxrfw. Xal jznnlv ttvwjgv.'

>>> decode('Ssw papvty unpervv fbpg cmi qavv mv zk pver-mdma iuziff.')
'Saw things clearer once you were in my rear-view mirror.'
>>> decode('Tal tpsl ba slhwyq lxrfw. Xal jznnlv ttvwjgv.')
'The past is always tense. The future perfect.'

Assignment

Example

Epilogue